This invention relates to a method of hardening prills and granules of inorganic salt compounds to reduce clumping and powder and dust formation.
A variety of inorganic salt compositions find extensive use in agriculture as well as in other applications. Examples of inorganic salt compounds which are used for such purposes include ammonium nitrate, di-ammonium phosphate, mono-ammonium phosphate, potassium nitrate, potash (potassium carbonate and potassium chloride) and various other inorganic salts.
To prevent caking and clumping and to obtain other desirable physical properties, it is customary to coat or otherwise modify these materials with one or more of a variety of different internal additives. Illustrative of these materials are magnesium oxide, various clays and mixtures of a variety of different inorganic salts. While these materials impart some beneficial properties, their use for these purposes has some serious disadvantages. For example, while magnesium oxide when applied to ammonium nitrate particulate improves hardness and stability, it is difficult to apply as it must be metered and handled as a solid. When incorporated in ammonium salts, magnesium oxide can also cause an undesirable release of ammonia. The application of magnesium oxide to inorganic salts can also result in increased hygroscopicity which may increase the likelihood of clumping and agglomeration which is highly undesirable.
Clay additives also present handling problems, due to their insolubility, which makes it necessary to handle them as solids and in the usual case, disperse them with difficulty, in the melt liquor used to produce the solid prill or granule of the inorganic salt being treated. Because they are inert and must be used in significant amounts, they also degrade the efficacy of the product because of dilution. Most of these modifiers also have an undesirable effect on the color of the product
Most inorganic salt mixtures used as modifiers for inorganic salts tend to be expensive, yield inconsistent results and require relatively high concentrations to be effective, thereby adversely affecting performance of the product, due to the effect of dilution. For example, ammonium sulfate is a well known hardening (anti-clumping and anti-dusting) agent for ammonium nitrate. Unfortunately it must be present in an amount of at least about 2% by weight of the total final composition to be effective. It also presents some very difficult manufacturing problems due to a necessity to form the sulfate salt in situ during the manufacturing process.
It has now been found that the use of a mixture of ammonium sulfate with certain naphthalene sulfonates (the expression xe2x80x9cnaphthalene sulfonatexe2x80x9d as used herein includes dimers and trimers of the described class of naphthalene sulfonates as well as monomeric naphthalene sulfonates) for improving the physical properties of inorganic salt granules and prills produces an unexpected and synergistic improvement in the physical properties of these materials. This invention also is directed to a process for preparing said mixture and for hardening inorganic salt granules and prills with said mixture by introducing an aqueous solution of ammonium sulfate and a naphthalene sulfonate into the melt liquor of the inorganic salt.
Illustrative of the inorganic salt compounds which can benefit from the invention are ammonium nitrate, di-ammonium phosphate, mono-ammonium phosphate, potassium nitrate, potash and complex fertilizer compositions. The invention is especially useful for the manufacture of fertilizer grade ammonium nitrate having superior hardness and stability properties.
The advantages achieved by the practice of this invention derive from the dramatic improvement in the physical properties of inorganic salt granules and prills which result from the introduction of a novel hardening mixture, comprising a naphthalene sulfonate compound and ammonium sulfate, into the inorganic salt. This novel mixture is a used in the form of an aqueous mixture of ammonium sulfate and one or more naphthalene sulfonate compounds of the formula (including dimers and trimers thereof): 
wherein M is ammonium or an alkali or alkaline earth metal; each R is independently, hydrogen or an alkyl substituent having from 1 to 12 carbon atoms; x is a whole number of from one to seven; and y is a whole number of from one to two with the provisos that the total number of carbon atoms contained in all of the R groups combined may not exceed 16 and the sum of x and y may not exceed eight. Illustrative of the useful naphthalene sulfonate compounds are sodium 1-methylnaphthalene sulfonate, potassium 1-methyl-3-methylnaphthalene sulfonate, potassium 1-isopropylnaphthalene sulfonate, calcium 4-ethylnaphthalene sulfonate as well as dimers, trimers and mixtures thereof. The useful naphthalene sulfonate compounds are easily prepared by sulfonation of naphthalene or an appropriately alkylated naphthalene compound followed by neutralization with ammonium hydroxide or an alkali or alkaline earth metal hydroxide compound. The addition of formaldehyde to the reaction mixture after sulfonation and before neutralization will yield the dimer or trimer form of the naphthalene sulfonate compound. The hardening mixture is preferably employed in an amount such that it will be present in an amount equal to between 0.05 percent to about 0.2 percent of the total weight of the final salt product.
It has also been found that by adding one or more optional alkali or alkaline earth metal benzene sulfonate compounds, which may have one or more alkyl substituents containing a total number of carbon atoms between three and 12, further improvements in the physical properties of the inorganic salt compound can be achieved. Illustrative of the useful optional alkyl benzene sulfonate compounds are sodium isopropyl benzene sulfonate, ammonium isopropyl benzene sulfonate, potassium di-isopropyl benzene sulfonate, magnesium xylene sulfonate, calcium 2-methyl-4-isopropyl-benzene sulfonate, sodium butyl benzene sulfonate, ammonium di-butyl benzene sulfonate, potassium hexyl benzene sulfonate, sodium octyl benzene sulfonate and sodium dodecyl benzene sulfonate.
The novel hardening additive of this invention is prepared by first dissolving a naphthalene sulfonate compound in water at a concentration of from about one percent to about 50 percent by weight. Ammonium sulfate is then added to the solution in an amount equal to from about 10 percent to about 70 percent by weight of the final solution. The optional benzene sulfonate compound may also be added at this time in an amount ranging from about one percent to about five percent by weight of the solution.
In a typical commercial production of solid inorganic salt prills or granules, using ammonium nitrate as an example, nitric acid and anhydrous ammonia are reacted together in a neutralizer to produce a slightly acid aqueous solution of ammonium nitrate at a concentration of about 82 to 83 percent by weight. It is preferred to incorporate the novel hardening additive of this invention into the inorganic salt compound melt at this stage. This is easily accomplished by the simple expedient of introducing the liquid additive mixture into a melt liquor of the inorganic salt. The amount added should be sufficient to produce an amount of the non-aqueous component of the additive equal, preferably, to from about 0.05 percent to about 0.2 percent by weight of the final inorganic salt product. If an optional benzene sulfonate is employed, the benzene sulfonate compound may be present in an amount of up to about 0.02 weight percent.
The aqueous solution is next passed through an evaporator to remove most of the water. This results in a xe2x80x9cmeltxe2x80x9d having an ammonium nitrate concentration on the order of 95 percent or higher. It is preferred to introduce the additive solution of this invention into the melt at this stage after the solution has passed through the evaporator. The higher density materials, normally those melts having a concentration nitrate salt greater than 98 percent are preferred for agricultural use as fertilizer.
In the next stage the melt is usually converted into prills or granules.
Prills are produced by spraying the melt downward from the top of a tower against a counter-current air stream within the tower cavity which cools the droplets to form solid prills as the melt crystallizes. The prills are collected at the bottom of the tower after which they may be further dried or cooled or coated in the conventional manner. The prills may also be coated at this stage with any of the conventional coatings such as clay or wax as an aid to prevent caking.
To prepare a granular product, the inorganic salt melt is simply sprayed onto smaller, usually recycled, inorganic salt particles which are being tumbled in for example, a rotating drum or pan. The melt cools and crystallizes on the surface of the particles. The now larger particles are sized and then cooled if necessary. The undersize particles are recycled back to the sprayer to serve as the substrate for further production of granules. Oversize granules are ground to a smaller size and are also returned to the sprayer.
Inorganic salt granules and prills prepared in the manner described above using the novel additive composition of this invention have an unexpectedly higher hardness, and thus superior physical properties, than salt granules produced by conventional methods despite using a smaller overall amount of additive than is employed in the conventional manufacture of inorganic salt granules. Each of the essential ingredients of the novel additive composition of this invention is capable of individually producing an increase in the hardness of the salt product. What is unexpected and surprising is that when combined, the resultant increase in hardness exceeds the combined improvement in hardness that is contributed by these materials when employed alone, by a substantial margin.